1. Field of the Invention
This invention relates to a method and apparatus for the real-time, non-destructive evaluation of welds formed by applying repetitive pulses of energy to the weld site. In particular, the invention is directed to monitoring stress wave emission signals during particular portions of the repetitive energy pulses to determine the acceptability of the weld.
2. Description of the Prior Art
The evaluation of adhesion bonds, welds or the like using real-time, non-destructive stress wave emission techniques is well known. For instance, U.S. Pat. No. 3,965,726 which issued on June 29, 1976 to S. J. Vahaviolos and is assigned to the instant assignee, describes the real-time evaluation of capacitive discharge welds by monitoring stress waves emitted from the weld site. Stress Wave Emissions (SWE's) may be defined as elastic waves which are characterized by low amplitude, short duration and fast rise time signals which are propagated in a structure as a result of an applied stress. As described in the aforementioned patent, the SWE energy emitted from the weld area during the solid-to-liquid phase transformation and the liquid-to-solid phase transformation are measured. The stress wave energy emitted during the liquid-to-solid phase transformation is subtracted from the stress wave energy emitted during the solid-to-liquid phase transformation and that value is compared with a predetermined value to determine the acceptability of the weld.
The foregoing technique has been found to be most successful in determining the acceptability of a weld. However, when the weld power is repetitively applied, as in AC welding, laser welding or the like, problems arise. During melting, a high SWE energy is to be expected which is reflective of the weld energy input as well as the size of the weld nugget; while during resolidification, the SWE energy should be low, which indicates few cracks and an acceptable weld. However, SWE signal energy measured during the period of application of the AC pulses in combination with the SWE signal energy measured during a predetermined time period after the AC weld energy has been applied, has been found to yield inconsistent results.
Accordingly, there is a need for real-time, non-destructive SWE techniques for determining the acceptability of a weld formed by a repetitively pulsed welding operation.